What are the best graphics cards for cycles?
Here's a spreadsheet that collates data from many users running the same benchmark scene. The thread with active discussion and a copy of the scene is at BlenderArtists, This is the best we can do without relying on opinion.

Decide for yourself which gfx card makes sense. 

Note: Different OS's, drivers and versions of Blender, can all significantly change render times. So you'll need to take that into consideration when comparing benchmarks.